1. Field of the Invention
The present invention relates to a steering control system of a vehicle such as an automobile.
2. Description of the Related Art
Recently, with regard to a steering system of a vehicle particularly of an automobile, the development has been conducted on a system that incorporates, as one way of further improving its functionality, a so-called variable steering angle conversion ratio mechanism that varies the conversion ratio (steering angle conversion ratio) of an operation angle of a steering wheel (steering wheel operation angle) to a wheel steering angle according to an operation state of the vehicle, without fixing the ratio of the steering wheel operation angle to the wheel steering angle to one to one. For example, the operation state of the vehicle may be represented by a vehicle speed. If the steering angle is prevented from abruptly increasing in response to an increase in the steering wheel operation angle by reducing the steering angle conversion ratio during high-speed operation, high-speed running of the vehicle can be stabilized. On the other hand, if the steering angle conversion ratio is increased during low-speed running, the number of turns of the steering wheel to the full lock position can be reduced. Accordingly, driving operation requiring a large steering angle such as parking a vehicle into a garage and parallel parking or pulling over the vehicle can be performed very easily.
A mechanism for varying the steering angle conversion ratio includes, as disclosed in Japanese Patent Laid-Open Publication No. 11-334604 for example, a type in which a steering wheel shaft is directly connected with a wheel steering shaft via a gear-driven transmission portion with variable gear ratio. However, the construction of this mechanism has a drawback that a gear ratio changing mechanism of the gear-driven transmission portion becomes complex. Therefore, a mechanism that rotates and drives the wheel steering shaft by a motor is suggested, for example, in Japanese Patent Laid-Open Publication No. 11-334628. Specifically, a final required wheel steering angle is calculated by computer processing based on the steering angle conversion ratio determined according to a steering wheel operation angle detected by an angle detecting portion and a vehicle operation status, and the wheel steering shaft which is mechanically disconnected from the steering wheel shaft is rotated and driven by the motor such that the calculated wheel steering angle can be obtained.
In such a steering control system, a rotation speed of a steering shaft driving motor is adjusted by PWM control based on a difference between an angular position of the wheel steering shaft (steering shaft angular position) and a target steering shaft angular position, so as to make the rotation of the wheel steering shaft follow the rotation of the steering wheel shaft. For instance, as the steering shaft angular position approaches toward the target angular position as the follow-up control proceeds, the rotation of the motor needs to be precisely controlled at low speed to avoid overshooting. On the other hand, in the case such as when the steering wheel is turned sharply, the motor for driving the wheel steering shaft is rotated at high speed such that the rotation of the wheel steering shaft keeps up with the operation of the steering wheel.